iommu/vt-d: support extended root and context entries

Add a new function iommu_context_addr() which takes care of the
differences and returns a pointer to a context entry which may be
in either format. The formats are binary compatible for all the old
fields anyway; the new one is just larger and some of the reserved
bits in the original 128 are now meaningful.

So far, nothing actually uses the new fields in the extended context
entry. Modulo hardware bugs with interpreting the new-style tables,
this should basically be a no-op.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>